1. Field of the Invention
The present invention relates to a manual valve for a hydraulic control system for an automatic transmission.
2. Description of the Prior Art
Automatic transmissions may be classified into two groups from the standpoint of arrangement of shift valves with respect to a manual valve. One group employs a so-called "series arrangement" wherein a 2-3 shift valve is coupled with an outlet port of a 1-2 shift valve which is coupled with a single outlet port of a manual valve such that the 2-3 shift valve is supplied with a line pressure only when the 1-2 shift valve is in its upshift state. The other group employs a so-called "parallel arrangement" wheren a 2-3 shift valve is coupled with an outlet port of a manual valve different from or same as that outlet port of the manual valve with which a 1-2 shift valve is coupled.
The "parallel arrangement" of the shift valves may allow the change-speed gearbox to interlock should at least one of the shift valves be sticked to its upshift or downshift state. Therefore, it is the conventional practice to design a hydraulic control system in such a manner as to prevent the occurrence of the interlock state of the change-speed gearbox.
Another design demand is that when the manual valve is set to a manual range like II range or I range, the pressure used for engaging the friction unit which plays a role in the second gear ratio must be temporally increased or boosted so as to ensure effective engine braking.
A known hydraulic control system for an automatic transmission disclosed in U.S. Pat. No. 3,667,323 issued to Namio IRIE on June 6, 1972 and assigned to Nissan Motor Company, Limited is designed to prevent the occurrence of interlock of the gearbox and to boost the line pressure during operation in the manual range for ensuring effective engine braking. However, a manual valve used in the known hydraulic control system is formed with six (6) outlet ports. Thus, this results in a cost increase in production of the manual valve, a cost increase in processing oil conduits and openings necessary for the valve body of the manual valve. Furthermore, it is not easy to find space sufficiently wide enough for a worker to machine ports or openings for the manual valve, restricting the design freedom of the hydraulic control system.